Local Urban Microclimate Research Articles

Machine learning-based surrogate models for fast impact assessment of a new building on urban local microclimate at design stage

The rapid urbanization introduces changes in the local urban microclimate. Many efforts have been paid on the impact assessment of building design on local microclimate. However, there is still a lack of efficient and accurate prediction method for assessing the impacts on local microclimate when making the design of individual buildings. Two complementary machine learning-based surrogate models are proposed, including an SVR-based local air temperature model and a LightGBM-based local wind velocity model. They are identified by evaluating and comparing eight alternative machine learning models, four for each model development. 200 sets of CFD simulation data corresponding to different building designs are used for the model training and validation. The results show that the developed surrogate models can dramatically reduce computation time (from over 5 h to less than a second for a single prediction) while keeping the same order of accuracy of CFD simulations for local microclimate prediction of individual buildings. It therefore facilitates the fast, comprehensive and accurate prediction of the impacts on the local microclimate at the early design stage of new construction and renovation of individual buildings, for designers to deliver preferred local microclimate and/or avoid unacceptable microclimate changes.

Utilization of typha-boards for preserving the historic wooden houses typical of the region in the Thai city of Chiang Mai (Thailand)

ABSTRACT The quality of life in the historic wooden houses typical of the city of Chiang Mai in northern Thailand is significantly reduced by climate changes and the intense air pollution during the Haze Season (time of slash-and-burn). The traditional indirect cooling system through strong natural infiltration ventilation in traditional houses is severely limited in its function due to changes in the local urban microclimate. The low comfort level leads to increased use of electric air conditioning, resulting in substantial energy consumption and the formation of urban heat islands and more and more often to the loss of historic Thai wooden houses by demolition.

Modelling urban dwellers’ indoor heat stress to enhance heat-health warning and planning

Due to climate change, the intensity, duration and frequency of heatwaves are likely to increase in the coming years. Excessive heat events can increase local urban heat island intensity affecting the health and wellbeing of urban dwellers vulnerable to heat stress. Heat-Health Warning Systems (HHWSs) have been developed to warn the public of impending heat events and to advise on preventable negative health outcomes. However, metrics upon which action triggers are made in HHWSs rely on reported critical outcomes, such as heat-related excess death. Thus, human exposure to heat is underestimated in current metrics and consequently, their capacity to prevent heat-related health risks remains uncertain, particularly indoors. This study investigates how indoor heat stress in urban dwellings at a city-scale can be modelled to enhance Heat-Health Warning and Planning. First, the effects of housing typologies on indoor thermal conditions are quantified in a local urban microclimate context. We then model the dynamic relationships between outdoor climate and indoor heat exposure to identify specific outdoor climatic thresholds as action triggers for alerting urban dwellers’ indoor heat stress. Based on urban microclimate data available for a city of Birmingham UK, a proof-of-principle study is presented. The result shows the presence of large variances in the heat-health action triggers across different housing typologies. This is further extended to consider the Birmingham climate projection scenarios provided by the UKCP18. Compared to the current UK Heat-Health Alert Service, we show how indoor heat stress warnings may look like and the implications for long-term heat-health planning.

Land use change impact on urban land surface temperatures: A GIS-supported satellite-based case study

This paper illustrates the use of GIS techniques and satellite data in order to analyze the impact of land use change on the local urban microclimate. Specifically, a case study is presented that concerns the city of Vienna. Thereby, satellite-based images were used to classify the city of Vienna into four zones toward the computation of land surface temperatures in two reference years. The classified maps were then statistically projected into the future, resulting in predicted land surface temperatures. The findings highlight the relationship between urbanization and temperature rise in the urban context. The study used data from Landsat 8 satellite in 2013 and 2020. Land cover maps were generated with QGIS for past and current conditions and future land cover maps were projected and corresponding land surface temperatures were predicted. The analysis of satellite data highlighted land surface temperature increase in the city of Vienna. This rise in land surface temperatures correlates with urbanization-driven change in land use and land cover.

Assessment of the potential of green wall on modification of local urban microclimate in humid tropical climate using ENVI-met model

This study assesses the effectiveness of the green walls for the modification of urban microclimate in different seasons of an urban academic campus in a humid tropical climate using the ENVI-met model. The hourly variation of the surface and ambient air temperatures of locations with differing underlying surfaces, such as asphalt (ASP), interlocked tiles (ILT), interlocked tiles with vegetation cover (ILTV) and open ground (OG) show significant variability implying the significance of shading effects. The ambient air temperature above the ILTV surface shows relatively lower temperatures during the winter (0.2–1.4 °C) and summer seasons (0.1–0.5 °C), compared to other substrates. Besides, the ambient air temperature above the ILTV surface during the afternoon hours (14:00–16:00 h) shows the maximum difference (compared to other surfaces) during the winter (1.3–3.1 °C) and summer seasons (0.8–2.1 °C). The results of the ENVI-met simulations indicate that the implementation of green wall building morphology could significantly reduce the ambient air temperature during winter (1.3–1.6 °C) as well as summer seasons (0.4–0.5 °C), but with differing intensities. Moreover, the green walls exhibit a maximum reduction of the ambient air temperature by 1.9 °C during the winter and by 0.8 °C during the summer. Although the results underscore the potential of green walls to reduce the ambient air temperature in a humid tropical climate, the study demonstrates the concerns associated with the choice of plant species in humid tropical climates with significant seasonality.

Impact of green walls on ventilation and heat removal from street canyons: Coupling of thermal and aerodynamic resistance

Green walls are considered a sustainable and environmental technology for improving the local urban microclimate. The presented wind tunnel measurements reveal the coupling effect of aerodynamic and thermal resistance of green walls in flat and steep street canyons in a simplified urban neighborhood model. The building and ground surfaces are set to thermal conditions at a series of Richardson numbers (Ri) ranging from 0 to 2.07. Green walls induce flow resistance due to their high skin friction and large volume-specific surface areas, which leads to the formation of a thick boundary layer near green walls, where hot air is trapped. These boundary layers weaken the convective heat transfer from building surfaces to the canyon, leading to an air temperature reduction in the canyon. The lower temperature reduces the strength of buoyancy, which, together with the aerodynamic resistance, results in the reduction of air flow speed, turbulence kinetic energy (TKE), and air ventilation in the canyon. The reduction of air ventilation is more significant in non-isothermal conditions, particularly for the steep canyon, where the air entrainment at the roof level is reduced by 32.4% in the isothermal condition, and air removal is reduced by up to 91.5% in the non-isothermal conditions (Ri of the steep canyon, Ris = 2.07). Green walls alter the flow structures by suppressing the strength of the downward flow, canyon-wide vortical flow, and ascending buoyant flow.

Fast generation of microclimate weather data for building simulation under heat island using map capturing and clustering technique

In this research, Urban Weather Generator (UWG) is utilized for UHI modeling in a center city area of Shenzhen based on urban morphologic data collected by an information-collection framework. The framework uses map capturing technique and unsupervised clustering for building type classification to gather input parameters for UHI model. The developed building type clustering technique, which adopted principal component analysis and Gaussian Mixture Model, is shown to have potential application in district-scale UHI modeling. Together with a differential-evolution-based calibration procedure, the proposed framework is implemented to the UWG modeling for the selected urban tissue. After calibration, it is found that the weighted sum of coefficient of variance for temperature and relative humidity has been reduced from 7.45 % to around 5.5 % compared with the uncalibrated model. The simulation results shows that the annual mean UHI index and max UHI index in center city Shenzhen is found to be 0.50 °C and 5.9 °C. After applying UHI effect to the typical meteorological year (TMY) data, the cooling degree day and heating degree day change by +12.60 % and −11.92 % compared with the original TMY. The developed method is computationally efficient and the morphed TMY data is a more accurate reflection on local urban microclimate.

Improving the design of an open auditorium: On the relationship between flow dynamics and building arrangement

The complexity of the built environment plays an essential role on the local urban microclimate. Therefore, this work aims to assess the impact of buildings arrangement on the overall turbulent flow dynamics. This study is conducted for the neighborhood of an open auditorium located in an urban area. A set of experiments were accomplished using a wind tunnel, together with a set of Computational Fluid Dynamics (CFD) simulations. Both the experiments and the CFD simulations were carried out for a set of meteorological conditions identified as prevailing in the auditorium area. The physical and numerical approaches were used to assess the wind patterns for the reference scenario. Furthermore, a series of mitigation measures was simulated to evaluate their effectiveness in reducing the wind speed at pedestrian level in the neighborhood. The overall results emphasize the role of the mitigation measures to reduce wind speed and potentially improve pedestrian wind comfort levels in the auditorium. The originality of this paper relies on the design and test of the set of proposed mitigation measures, through the combination of complementary tools, as well as the expertise of distinct practitioners, and their insights regarding their effectiveness.

Improving pedestrian micro-climate in urban canyons: City Center of Alexandria, Egypt

Due to the accelerated pace of urbanization, the study of the local urban microclimate has acquired significance. A detailed and accurate prediction of outdoor ventilation and thermal comfort is required for a healthy environment, particularly in densely built areas, due to the complex interaction of physical phenomena across different length and time scales. Engineering approaches are critical for solving the issues of wind discomfort. These approaches can however not be applied independently of the architectural context. An interdisciplinary approach can bridge the gap between architectural design and engineering wind-comfort solutions in a coastal city of developing countries with a temperate climate. The current study aims to evaluate the effect of urbanization on an asymmetrical historical canyon in the city center of Alexandria, Egypt. Challenges and limitations are imposed regarding applying retrofitting techniques to improve pedestrian comfort without changing the historical features of these areas.This paper provides a Computational Fluid Dynamics (CFD) simulation of a residential district in the modern city of Alexandria which contains the library of Alexandria and the two oldest paved roads in the world which still in use until today. The 3D steady Reynolds-averaged Navier-Stokes (RANS) equations and the standard k-ε model are used to calculate the aerodynamic flow field. These computational settings were validated with previously published wind-tunnel results. The indices of thermal comfort were calculated using a Predicted Mean Vote (PMV) model utilizing Custom Field Functions (CFF). On-site measurements of air velocity, temperature, and humidity were conducted to validate the thermo-flow characteristics with the numerical simulation and they were in a good agreement.To reach the comfort level which contributes to a pedestrian-friendly environment, it brings a range of measures that covers qualitative design guidelines on street and building level to mitigate wind discomfort and excessive solar access in urban canyons. One of these measures presents (CFD) simulations for comparing the model of the modern city with a visualized model of the ancient city of Alexandria at the end of Cleopatra's reign. This comparison aims to give a close vision on how urbanization has influenced the aerodynamics and thermal comfort in this region. The current study helps urban planners and architects to enhance the current situation for better urban microclimate at the pedestrian level, without setting up new developments during hot summer days.

Role of Species and Planting Configuration on Transpiration and Microclimate for Urban Trees

Research Highlights: To demonstrate the effectiveness of configuration modes and tree types in regulating local microclimate. Background and Objectives: Urban trees play an essential role in reducing the city’s heat load. However, the influence of urban trees with different configurations on the urban thermal environment has not received enough attention. Herein we show how spatial arrangement and foliage longevity, deciduous versus evergreen, affect transpiration and the urban microclimate. Materials and Methods: We analyzed the differences between physiological parameters (transpiration rate, stomatal conductance) and meteorological parameters (air temperature, relative humidity, vapor pressure deficit) of 10 different species of urban trees (five evergreen and five deciduous tree species), each of which had been planted in three configuration modes in a park and the campus green space in Xi’an. By manipulating physiological parameters, crown morphology, and plant configurations, we explored how local urban microclimate could be altered. Results: (1) Microclimate regulation capacity: group planting (GP) > linear planting (LP) > individual planting (IP). (2) Deciduous trees (DT) regulated microclimate better than evergreen trees (ET). Significant differences between all planting configurations during 8 to 16 h were noted for evergreen trees whereas for deciduous trees, all measurement times were significantly different. (3) Transpiration characteristics: GP > LP > IP. The transpiration rate (E) and stomatal conductance (Gs) of GP were the highest. Total daily transpiration was ranked as group planting of deciduous (DGP) > linear planting of deciduous (DLP) > group planting of evergreen (EGP) > linear planting of evergreen (ELP) > isolated planting of deciduous (DIP) > isolated planting of evergreen (EIP). (4) The microclimate effects of different tree species and configuration modes were positively correlated with E, Gs, and three dimensional green quantity (3DGQ), but weakly correlated with vapor pressure deficit (VpdL). (5) A microclimate regulation capability model of urban trees was developed. E, Gs, and 3DGQ could explain 93% variation of cooling effect, while E, Gs, VpdL, and 3DGQ could explain 85% variation of humidifying effect. Conclusions: This study demonstrated that the urban heat island could be mitigated by selecting deciduous broadleaf tree species and planting them in groups.

Quantifying Impacts of Urban Microclimate on a Building Energy Consumption—A Case Study

This paper considered an actual neighborhood to quantify impacts of the local urban microclimate on energy consumption for an academic building in College Park, USA. Specifically, this study accounted for solar irradiances on building and ground surfaces to evaluate impacts of the local convective heat transfer coefficient (CHTC), infiltration rate, and coefficient of performance (COP) on building cooling systems. Using computational fluid dynamics (CFD) allowed for the calculation of local temperature and velocity values and implementation of the local variables in the building energy simulation (BES) model. The discrepancies among the cases with different CHTCs showed slight influence of CHTCs on sensible load, in which the maximum variations existed 1.95% for sensible cooling load and 3.82% for sensible heating load. The COP analyses indicated windward wall and upstream roof are the best locations for the installation of these cooling systems. This study used adjusted infiltration rate values that take into account the local temperature and velocity. The results indicated the annual cooling and heating energy increased by 2.67% and decreased by 2.18%, respectively.

Mitigation potential of green structures on local urban microclimate using ENVI-met model

ABSTRACTEstablishing green roofs has been used as a method to mitigate urban environmental quality problems. In this study, the effect of Tehran Nature Bridge green roof as a sustainable structure on its surrounding microclimate is investigated using urban microclimate model, ENVI-met, on a hot summer day in July 2015. For this purpose, two simulation scenarios were generated to assess the impact of extensive and intensive green roofs on some climate variables. A comparison was made between the scenarios at two different heights, 1.5 and 40.5 m and two times including 10 am and 4 pm. Based on the predicted results, the second scenario showed a 10.47% increase in relative humidity (RH) compared to the first scenario on bridge level at 10 am. In the second scenario, air temperature dropped by 0.8°C at 10 am compared with the first scenario, which is equivalent to 2.87%. The presence of intensive green roof caused a decrease in wind velocity on bridge level by 1.75 m/s and changed its direction. Furthermore, green roof also influenced velocity and direction of the wind, RH, and air temperature in different ways at ground level. Overall, green roof altered climatic parameters such as air temperature, RH, and wind velocity and direction at both bridge and ground level.

Simulations of local heat islands in Zürich with coupled CFD and building energy models

Due to the urban heat island (UHI) effect the air temperatures in urban areas are most of the time higher compared to the temperatures in rural areas. Further the wind speeds are lower due to wind sheltering. In the past decades cities and therefore also the urban heat islands have been continuously growing. The local microclimate has a strong impact on the energy demand of buildings and the human comfort and health. Only few guidelines exist for urban planners to mitigate UHI effects or its impacts. This study aims at simulating the effect of new buildings on the local urban microclimate for a building site in Zürich (Switzerland). One-way coupled CFD and building energy simulations are conducted to determine the increase in air and surface temperatures due the presences of new buildings. The daytime local air temperatures for weather conditions with high ambient air temperatures, which are considered to be most critical for thermal comfort, are studied. The results show that the formation of local hot spots strongly depend on the building geometries, building materials, the strength of buoyancy and the wind directions and wind speeds.

Quantify Impacts of Local Urban Microclimate on Local Airflow Patterns

This study aims to quantify the impacts of the local urban microclimate on the local airflow patterns, including air temperature and Convective Heat Transfer Coefficients (CHTCs). Consideration of the microclimate require taking into account influence of the local outdoor air temperature, velocity, and solar irradiance. Accurate representation of these variables plays an important role in the assessment of building energy consumption patterns in urban areas. This study uses Computational Fluid Dynamics (CFD) with implementing the local solar irradiances on the building surfaces to calculate local airflow patterns. The performed simulations indicate accounting for the local urban microclimate allows potential implementation of urban scale CHTCs in the building energy simulation models. Accurate estimation of local microclimate provides crucial inputs for the building energy models.

Influence of morphologies on the microclimate in urban neighbourhoods

In the past decades the portion of the population living in urban areas has continuously increased. Due to the high building density, the microclimate in urban areas changes significantly compared to rural areas. The temperatures measured in urban areas are higher compared to the rural temperatures due to the urban heat island (UHI) effect. Furthermore, the longwave and solar radiation exchanges are influenced by shadowing, reflections between buildings and reduced sky view factors. The local urban microclimate has an influence on the energy demand of buildings and on human comfort and health in urban areas. In summer the human comfort in urban areas can decrease due to higher air and surface temperatures and lower wind speeds compared to rural areas. The local urban microclimate is difficult to predict because of the complex interaction of physical phenomena across a large range of time and length scales. Few guidelines exist for architects to mitigate UHI effects or its impacts. The aim of this paper is to model the urban microclimate with CFD and building energy simulations and to investigate in detail the influence of different urban building morphologies on the urban microclimate. This approach and the results of this study can be used to find measures to mitigate the UHI effect. The results show that building façade surface temperatures are mainly influenced by the distance between buildings. For urban morphologies with similar surface temperatures, the air temperatures can still strongly vary due to different wind flow patterns causing different rates of removing heat by wind.

Effects of urban trees on local outdoor microclimate: synthesizing field measurements by numerical modelling

In this study, we investigated the effects of trees on the local urban microclimate and human thermal comfort under different local weather conditions, in a small urban area in Assen, the Netherlands. In both summer and winter, continuous air temperature and relative humidity measurements were conducted at five selected sites having obviously different environmental characteristics in tree cover. Measurements demonstrated that in summer the microclimatic conditions at each observation site showed significant differences. The cooling effects of trees on clear and hot days were two times higher than on cloudy and cold days. In winter, air temperature was slightly reduced by the evergreen trees, and weather conditions did not cause a notable change on performance of trees on the microclimate. ENVI-met, a three-dimensional microclimate model was used to simulate the spatial distribution of temperature and humidity. After selecting representative days, we simulated the study site as it currently is and for a situation without trees. Spatial differences of trees’ effects were found to vary strongly with weather conditions. Furthermore, human thermal comfort is indicated by the Predicted Mean Vote model. During the hottest hours, trees improved the thermal comfort level via reducing ‘very hot’ and ‘hot’ thermal perception by about 16 % on clear days and 11 % on cloudy days. Generally, our findings demonstrate that urban microclimate and human thermal comfort convincingly varies in close geographical proximity. Both are strongly affected by the presence of local trees. Weather conditions play an important role on the trees’ performance on the summer-time microclimate.

Experimental study of the urban microclimate mitigation potential of green roofs and green walls in street canyons

This paper presents the design and the results of a reduced-scale experimental bench to study the effect of green roofs and green facades on local urban microclimate. An analysis of experimental data was carried out on three street canyons: one reference street, one street between two buildings with green roofs and one with a green wall to the west. The results show a hygrothermal effect of green envelopes on buildings on urban heat island mitigation. In the detailed analysis of these green coating techniques, we highlighted that green facades modify strongly the radiative balance of the street and improve the hygrothermal comfort in urban canyons through reducing the overheating in the street in hot summer days.

Addressing the Challenge of Interpreting Microclimatic Weather Data Collected from Urban Sites

This paper presents some installation and data analysis issues from an ongoing urban air temperature and humidity measurement campaign in Hangzhou and Ningbo, China. The location of the measurement sites, the positioning of the sensors and the harsh conditions in an urban environment can result in missing values and observations that are unrepresentative of the local urban microclimate. Missing data and erroneous values in micro-scale weather time series can produce bias in the data analysis, false correlations and wrong conclusions when deriving the specific local weather patterns. A methodology is presented for the identification of values that could be false and for determining whether these are “noise”. Seven statistical methods were evaluated in their performance for replacing missing and erroneous values in urban weather time series. The two methods that proposed replacement with the mean values from sensors in locations with a Sky View Factor similar to that of the target sensor and the sensors closest to the target’s location performed well for all Day-Night and Cold-Warm days scenarios. However, during night time in warm weather the replacement with the mean values for air temperature of the nearest locations outperformed all other methods. The results give some initial evidence of the distinctive urban microclimate development in time and space under different regional weather forcings.